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Notizen: Summary The introduction made mention of features necessary to consider in design of the most suitable water-current respirometer. We feel our apparatus suitably meets these requirements where other presently used devices are unsatisfactory in one way or another. Size has been reduced to attain a volume of 200 ml, sufficient to complete an experiment with, for example, one large caddis larva in two hours. If less time is required, the apparatus can be miniturized further. (We have constructed several units by drilling or milling all components, except current tube and animal chamber, into plexiglass blocks. Although one must use a smaller spin bar to achieve lower velocities, other functional characteristics of these 85 ml units are identical to the larger ones). The animal chamber may easily be modified to accomodate a wide variety of lotic invertebrates and a series of these respirometers are easily and rapidly set up, whatever the modification. Magnetic current induction eliminates parafin oil, gaskets or gears, etc., which are messy or difficult in construction and are questionable when considering oxygen contamination. Currents are reasonably uniform and non-turbulent in all parts of the animal chamber as they pass over the experimental animal(s) from one direction. By contrast highly turbulent flow is difficult to quantify and interpret; and current-swirls offer various velocities with slight lateral movement of the organism, thus making it quite impossible to know the effect of any given velocity. The size and control of the driving motor and impeller magnets determine the constancy of current velocity, variety of currents available and in part the ultimate velocity attainable. Magnetic stirrers fail in these respects as well as with standardization between units. Our design is simple enough that it can be duplicated exactly, allowing equal rpm to result in equal current velocity in different units. The design makes oxygen electrode monitoring feasible, because water movement can be very fast in the outer swirls of the current box, accurately controllable and constant. Accuracy of measurement must be questioned in situations where these capabilities are u navailable. While the greatest velocity possible previously in a respirometer suitable for use with invertebrates was approximately 20 cm/sec, velocities of 100 cm/sec are attainable in our apparatus. According to Nielsen (1950), 50 cm/sec, may be classed as the lower limit of swift flowing water. Rarely will current velocity exceed 115 cm/sec even in rushing mountain streams. A device producing such velocities would obviously aid in answering a number of questions concerning effects of the current.